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Joel 20th November 2002 06:52 PM

Interstage - is it pointless?
After posting a question last month about the hammond 850Q, and not getting much response, I did some research on my own. I really was hoping to put together a transformer coupled amp, but it seems that there is almost nothing to be gained by doing so - and a lot to be potentially lost.
Am I wrong?
They seem to: a) cost more than many output transformers, b) have limited bandwidth - even the pricey ones, and c) have many restrictions as to input impedence, capacitance, plate current of the driver tube, plate resistance of the source, etc... Not to mention all the "specs" I see at most manufacturer's sites are for an unloaded condition. Is that a valid measurement?
None of these restrictions would apply if I were to simply rc couple it. Bandwidth can be as high as I want it to be.

SHiFTY 20th November 2002 07:06 PM

I have always wondered this myself- surely a capacitor, with its slight phase changes at different frequencies, would still have an order of magnitude less distortion than a non-linear device like a transformer?

One transformer in the circuit is probably enough! If you use a large enough coupling cap, the bandwidth would be very large.

Why then, apart from the simplicity factor, do people use them?

And why don't more people use direct coupling?

SY 20th November 2002 07:12 PM

Shifty, the usual reasons are fashion and desire to introduce euphonious (to the designer's ear) distortion and/or bandwidth limiting. Designs where the output is intended to accurately reflect the input do not generally use interstage transformers. Designs intended to "prettify" the sound often do.

Direct coupling in tube circuits can be quite successful, if thought through properly and set up with sufficient adjustments to reduce the impact of tube variations.

dhaen 20th November 2002 07:14 PM

Reason 1:
To drive a low mu triode, running at high voltage, the peak-to-peak drive required can exceed 400 volts. Try getting that out of a driver stage - and stay linear.
The transformer, even when 1:1 (the best bandwidth kind) allows twice the voltage swing.
NB An anode choke achieves the same thing in terms of drive voltage.

Reason 2:
To provide a linear load for the driver valve. This reduces distortion.

Reason 3:
To provide a low impedance AC source. This reduces distortion.

NB An anode choke in the driver achieves these things also.

These help satisfy the low natural distortion requirements for a no-feedback amplifier.
Indeed, global feedback would be most unwise with 2 transformers in the loop.


Joel 20th November 2002 07:34 PM

John, I don't want to see the triode that needs a grid voltage of minus 200volts!!! Yikes! :eek:

As far as your point that the IT provides lower distortion - the few times distortion measurements were given for IT's, it's always around 1.5% - which is about what a typical RC stage is, before NFB. Isn't it?
The higher swing argument is easy to understand.

dhaen 20th November 2002 07:49 PM


The distortion argument is that the transformer allows the preceeding and following stage to run at lower distortion.

I don't have any figures for the actual distortion within the transformer; where did you find those figures?

IMO a capacitor and 2 resistors on their own create practically no distortion. But I'd better keep quiet about that;)


Joel 20th November 2002 07:58 PM

One source was the Hammond website. I did so much surfing, I can't remember the others offhand.:hypno1:

dhaen 20th November 2002 08:05 PM


I'll measure mine tomorrow and post the result.
It'll depend a lot on level and current, I think.


tubesguy 20th November 2002 08:08 PM

I'll add Reason #4:

An interstage tranny secondary provides a low DCR path to ground as compared to a grid resistor on the driven (ie, output) tube. Less concerns with grid current in that tube.

In addition, the secondary of the interstage can be a fine place to inject the voltage required for fixed bias of the output tube, if your tastes run to that method. - Pat

fdegrove 20th November 2002 09:06 PM


You knew I had to chip in right?;)


It'll depend a lot on level and current, I think.
Yep,the more current these need to swing the worse distorsion levels get.


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